This invention relates to gas flow measurements and, more particularly, to the correction of a measured unit volume of gas flowing through a conduit to a base volume at given base conditions of base pressure and base temperature.
Gas is a compressible item, the volume of which changes as a function of temperature and pressure, in accordance with well known physical laws. Because gas is a compressible commodity, the buyer and seller of this commodity must agree upon the same conditions. Thus, to distribute and sell gas that is exposed to varying conditions of temperature and pressure, calculations must be made to convert the measured gas flow volume V.sub.f in terms of cubic feet at varying conditions of temperature T.sub.f and pressure P.sub.f, to a standard cubic feet volume V.sub.b at specified, previously agreed upon base temperature T.sub.b and base pressure P.sub.b.
The basic gas law equation of state is EQU PV=WRTZ (1)
where:
P is pressure PA1 V is volume PA1 W is mass PA1 R is gas constant PA1 T is temperature PA1 Z is compressibility. PA1 V.sub.f is the measured uncorrected volume PA1 P.sub.f is the flowing gas gage pressure PA1 P.sub.a is atmospheric pressure PA1 P.sub.b is base pressure PA1 T.sub.b is base temperature in degrees Rankine PA1 T.sub.f is the temperature of the flowing gas in .degree.R PA1 F.sub.pv is the supercompressibility factor which is equal to ##EQU2## The difficult part of calculating base volume in accordance with equation (2) is to determine the supercompressibility factor which is a function of the flowing temperature and pressure as well as the specific gravity and the composition of the gas being measured. One way of determining the supercompressibility factor is to utilize tables such as those set forth in the "Manual for the Determination of Supercompressibility Factors for Natural Gas", PAR Research Projects NX-19, published by the American Gas Association. However, if it is desired to automate the correction of measured gas flow, it is difficult and expensive to utilize tables. Alternatively, it is possible to utilize a series of equations to calculate the supercompressibility factor. U.S. Pat. No. 4,173,891 discloses such an automated system including a microprocessor for repetitively calculating the supercompressibility factor. The method employed by the patented system includes a plurality of computing steps for each calculation of the base natural gas flow. During each computing step, an initially approximated value of the supercompressibility factor or the previously calculated value is used to calculate an indication of the base natural gas flow, each computing step being insufficient to recalculate the supercompressibility factor, this calculation taking a plurality of steps. Therefore, the disclosed system has the disadvantage that a relatively large amount of time is required each time the supercompressibility factor is to be calculated. In fact, it requires five input meter pulses for a complete calculation to be performed. Another disadvantage of the system disclosed in this patent is that the calculations are performed utilizing floating point arithmetic, which requires a large amount of memory capacity, increasing the cost of the system hardware.
When dealing with a simple gas, such as N.sub.2 or O.sub.2, the classic gas laws serve very well, and Z may not be needed. However, when there are mixtures of gases and complex hydrocarbons, it has been found that Boyle's and Charles' laws are in error. Fuel gases tend to be easier to compress, up to around 2000 psig, than these laws would suggest. Above this pressure, the trend is reversed. The exact values are functions of the pressure, temperature and the gas composition. The difference between the classic gas laws and the complex gas compression is called compressibility, or Z.
From equation (1), the following relationship between base and flowing conditions may be derived: ##EQU1## where: V.sub.b is base volume
It is therefore an object of this invention to provide apparatus for measuring gas flow.
It is a further object of this invention to provide such apparatus which automatically corrects the measured gas flow to predetermined base conditions of temperature and pressure.
It is another object of this invention to provide such apparatus wherein the corrected values are efficiently calculated.
It is yet another object of this invention to provide such apparatus wherein it is relatively easy to make adaptations for different base conditions and gas composition.
It is still another object of this invention to provide such apparatus which can be utilized at remote meter locations without the necessity for connection to an external source of power.